Microwave absorbing wall element

ABSTRACT

A microwave absorbing wall element to be used, for example, in an anechoic chamber, which element comprising a polygonal ferrite material plate formed to have complemental stepped edges in section at its periphery, and a conductive plate lined on the back surface of the ferrite material plate. As the elements are butted together with each other, the stepped edges of adjacent butted elements will engage with each other so as to block joint slits along each periphery.

United States Patent [72] Inventor Saline Fukui 234, l'llgasbi Okubo Z-chome. Shinjuku-ku, Tokyo, Japan [21] Appl. No 865,231 [2 2] Filed Sept. 24, 1969 [451 Patented July 27, 1971 COIIflIIIJIflOlI-lll-Plfl of application Ser. No. 699,522, Jan. 22, 1968, now abandoned. Priority Apr. 1, 1967 3] Japan l 3 l I 42/26784 [54] MICROWAVE ABSORBING WALL ELEMENT 3 Claims, 10 Drawing Figs.

[52] U.S.Cl. 343/18A [51] Int. Cl. H01q 17/00 [50-] Field of Search... 343/18 A [56] References Cited UNITED STATES PATENTS 938,430 10/1909 McQuern 52/593 X 3,308,462 3/1967 Gluck 343/18 A Primary Examiner-T. H. Tubbesing Attorney-Pierce, Scheffler 8!. Parker sum 2 or 2 VSWR VSWR

"-- FREQUENCY INV ENTOR Sakae Fukui TTO R N EY S 1 MICROWAVE ABSORBING WALL ELEMENT This is a continuation-in-part of application Ser. No. 699,522, filed Jan. 22, 1968, now abandoned.

This invention relates to microwave absorbing walls using ferrite plates.

The microwave absorbing wall according to the present invention is used, for example, in an anechoic chamber required to measure the characteristics of such devices for electromagnetic waves as antennas.

In measuring the characteristics of an antenna or the like,,it is desirable that there should be no electromagnetic wave at all from others than the thing to be measured. Therefore, it is usual to carry out the experiment on the roof or in the field instead of carrying it out in the room. However, even in such places outside the room, waves reflected by the buildings and the ground may disturb the experiment-or the weather may prevent the experiment from being carried out. Therefore, microwave absorbing walls have been invented andanechoic chambers in which such absorbing walls are put on the peripheral walls have come to be utilized.

However, recently, with the riseof the availability of such microwave absorbing walls, the frequencies to be used have come to be lower. The thickness of such microwave absorbing wall is different depending on thereflection coefficient andproposed frequency band of the wall. Thus, the wall thickness required for frequencies low in the V.H.F. degree will be more than 1 meter. Further, there is a drawbackthat, when the characteristics of the microwave absorbing wall are to be improved within the same frequency band, the required wall thickness will have to be increased.

An object of the present invention is to provide a microwave absorbing ,wallelem ent which can be easily manu factured at low cost and the thickness of a microwave absorbing wall comprising the elements will be so small as to be about one-sixtieth of the free space wave length.

Another object of the present invention is to provide at microwave absorbing wall elementof less reduction in characteristics at the junctions at peripheral edges of the elements with each of adjacent elements and manufacturing of the microwave absorbing wall comprising the elements will be easy.

Other objects and advantages of the present invention will become clear upon reading the following disclosures and upon referring to the accompanying drawings.

FIGS. lAand 1B show anembodiment of theferrite microwave absorbing wall element according to the present invention, FIG. 1A being a front view and FIG. 1B is a side view of the embodiment.

FIGS. 2A and 2B show another embodiment of the present invention, FIG. 2A being a front view and FIG. 2B is a side view.

FIG. 3 is a cross-sectional view on the line Ill-Ill in FIG. 2A.

FIGS. 4A and 4B show another embodiment of the present invention, FIG. 4A being a front view and FIG. 4B is a side view.

FIG. 5 shows a perspective view of a microwave absorbing wall which comprising the elements as shown in FIG. 1.

FIG. 6A and 6B are views illustrating comparative operating characteristics of a microwave absorbing wall utilizing ferrite plates of conventional construction and a wallstructure in accordance with the present invention.

It should be understood that, while the present invention has been explained with reference to the preferred embodiments, it is intended not to limit the present invention to the particular embodiments as shown, but rather to cover all alterations and equivalent arrangements to be included in the scope and spirit of the invention as claimed.

Ina conventional microwaveabsorbing wall in which is used a magnetic material, if the specific permeability and specific permittivity of the material are respectively 1 its normalized wave impedance will be given by VIP/R1 It has been know, therefore, if in, and e, are'respectively taken to be larger than 1 and it the normalized wave impedance of such material will be 1 so as to provide the microwave absorbing action and, thus, the material will be utilized for the wave absorbing wall. However, no material satisfying this condition over a wide" frequency range has been obtained through researches so far. It has been difficult to realize a wave absorbing wall based on this idea. Further, in conventional microwave absorbing walls, structure having a short circuit plate placed at a distance of A/4behind a dielectric resistance film or lamina has been employed. In such a structure, there has been a disadvantage that the frequency characteristics are inherently reduced. On the'other hand, the present invention is to provide a microwave absorbing wall which utilizesa magnetic material and'having-a novel structure of anew idea. According to the present invention, a magnetic resistance material of a magnetic resistivity (to be explained later) is made a lamina of a thickness 1, the'relation of is satisfied, the backsurface is lined with a conductive plate so as to be short-circuited and, further, the material is formed in an element having apolygonal shape and its peripheral edges are stepped to have an overlappingand interlocking engagement with adjacent elements. As seen from this formation, as different from the case of microwave absorbing walls utilizing the dielectric resistance film, it is not necessary to provide the short circuit plate at the wave length distance of M4 behind the element. Therefore, it is a featurethat the reduction of the frequency characteristics bythe back surface impedance need not be considered. It will be also apparent that, with the provision of the stepped edges, the microwave absorbing wall formed of such element as above according to the invention is efi'ective in absorbing any waves that penetrate through slits between butted elements against each adjacent ones, which will be absorbed by the stepped edges.

With reference to 7,. the feature of the magnetic material is generally given by the following formulas:

Therefore, the matching condition of the formula is as follows:

and l f gcah wherein m=21rf (an angular velocity), f is a frequency and it is a wavelength.

The condition formula is reverse to the conditionof using the conventional magnetic material. It is necessary that the material should have such characteristics that the so-called permeability p in the ordinary sense of the word is as small as possible tobe close to 1 and the loss term p varies in inverse-proportion to the frequency (or in proportion to the wavelength).

Therefore, the microwave absorbing wall can be formed by making the plates with the material as referred to in the above, which plates have a thickness determined by the above formula and a polygonal shape, of which periphery is partly or all formed in a stepped section, lining the rear surface of the each plate with a conductive plate or foil and spreading the plates closely one another all over the required space.

In FIGS. IA and IB, 1 is a front section of the regular hexagonal shape and 2 is a rear section likewise of the regular hexagon identical to the shape of the front section I. Both sections 1 and 2 are so formed in one body as to be offset, so that inward .and outward stepped parts 3 will be provided at peripheral edges. Therefore, when a plurality of the wall elements respectively thus formed are butted together against each other in order to establish the microwave absorbing wall, outward stepped parts of respective elements will engage with inward stepped parts of respective adjacent elements. 4 is a conductive plate provided on the surface of the rear section 2. It should be apparent that the microwave absorbing wall thus established will have a flat and plane front surface of successive hexagons of the ferrite plates and likewise a flat and plane rear surface of successive hexagons of the conductive plates, and that respective hexagons on each of the surfaces are arranged in a offset relation to each other, as partly shown in FIG. 5. Therefore, even though the wall is formed of a number of plate elements successively butted against each adjacent ones at their peripheral edges, the wall has no penetrating slit at the butted joints through the wall and, therefore, the wall provides a perfect absorbing action allover the wall surface.

In FIGS. 2A and 2B, 1 is the front section of the regular hexagonal shape and 2 is a rear section. The rear section 2 in this embodiment is formed in an irregular shape, which will provide the stepped parts 3. As illustrated, the stepped parts 3 are formed in such manner that, in this embodiment, lower left and right side parts 5 and upper side 8 of the rear section 2 are projected, and upper left and right side parts 6 and lower side 7 are internally recessed. In other words, the element is provided with a regular hexagonal front section having at its respective rear side peripheries three projected sections 5 and 8 and three recessed sections 6 and 7, alternately, so that each projected section will engage with each recessed section of an adjacent element when a plurality of such elements are butted to each other; thereby the thus fonned wall surface will be a plane and flat surface of successive pattern of the hexagon and each joint slit will be blocked at its rear by each rear side projected section.

FIGS. 4A and 4B show an embodiment in a regular square shape, wherein I is the front section, 2 is the rear section, 3 are the stepped parts and 4 is the conductive plate.

FIG. 5 shows a certain number of the elements shown in FIG. I as being set into one another.

As an example, the microwave absorbing wall according to the present invention was formed by lining regular hexagonal ferrite plates of, for example, 1 mm. thick and 50 mm. maximum size onto aluminum plates with a bonding agent and arranging them closely one another. Thus constructed microwave absorbing wall has shown an excellent characteristics of a standing wave ratio p l 3 over the frequency of 4002000 me.

The operation of the microwave absorbing wall according to the invention, comprising the ferrite plates of thickness (placed adjacent the conductive plate, will be explained as follows:

In the case when the relative permittivity and permeability of ferrite material are e and ;i.-,=p.-, 'ju-, "respectively, normalized input impedance will be z= tanh il-r) when t 7 and A 1 and, thus, the matching condition is satisfied.

In order that the matching condition is satisfied over a wide range of frequency, it is desired that, from the above condition, the loss term 1., will be increased in proportion to the wavelength (inverse proportion to the frequency) as follows:

in which v is light velocity. On the other hand, it is known that the ferrite has such a characteristic that its will become smaller substantially in reverse proportion to a higher frequency than a magnetic resonant frequency specific to the ferrite. Thus, it is possible to form a microwave absorbing wall which is capable of operating in a remarkably wider range of frequency, by utilizing such characteristic. The present invention has been suggested on this particular basis. It should be here noted that the present invention is characterized in that the invention utilizes the loss of ferrites, whereas the ferrites have been conventionally used in various parts of communicating devices having a less high frequency loss.

Since the present invention utilizes the magnetic characteristic of ferrites in a higher frequency, as described in the above, the ferrite tile plates have to be arrayed in such manner that there will occur no leakage of magnetic flux between each of the butted edges of the ferrite tiles so as not to reduce seeming permeability thereof. Such reduction tends to deteriorate the microwave absorbing characteristics of the wall, as diagrammatically shown in FIG. 6A. As will be seen from the drawing, even a small clearance of I mm, will greatly affect the characteristics, in the case of using tiles of straight cut edges. It is almost inevitable, however, that such a small clearance is involved in actual arraying of a number of tiles one by one. Thus, the present invention has been suggested as a main aspect to provide a structure of the ferrite tiles which enables it possible to have the magnetic flux always passed through each of adjacent tiles, even when there is caused a clearance to be present between butted edges at front side of the tiles.

For this purpose, the ferrite tiles according to the present invention are provided at their rear side a stepped projection and a complementary stepped recess, so that each projection of a tile will be registered in each recess of an adjacent tile. With this structure, the present invention establishes such a feature that, even in the case where the clearance is mechanically caused to present between the butted joint of ferrite tiles, the microwave absorbing characteristics of the wall is less affected, as will be seen in FIG. 6B.

In the drawing, it is seen that even the distance d between respective edges at front side of adjacent tiles is increased to, for example, I mm. and to 2 mm. voltage standing wave ratio is not remarkably deteriorated comparing with the ratio at the time of the clearance is zero.

What I claim is:

1. In a microwave absorbing wall structure, the combination comprising an assembly of flat polygonal plates of like configuration arranged in side-by-side relation, each said plate consisting of a front section and a rear section, and an electrically conductive plate adhered to said rear section, and the side edges of said front and rear sections of each plate being offset from one another thereby to establish inwardly and out wardly stepped portions which interfit with the complementary stepped portions of the other plates adjacent thereto and effect an overlap of all joints formed between the abutting side edges of the front sections of said plates in relation to the joints formed between the abutting side edges of the rear sections ofsaid plates.

2. A microwave absorbing wall structure as defined in claim I wherein said plates have a regular hexagonal configuration and three adjacent sides of the front section thereof are offset 

1. In a microwave absorbing wall structure, the combination comprising an assembly of flat polygonal plates of like configuration arranged in side-by-side relation, each said plate consisting of a front section and a rear section, and an electrically conductive plate adhered to said rear section, and the side edges of said front and rear sections of each plate being offset from one another thereby to establish inwardly and outwardly stepped portions which interfit with the complementary stepped portions of the other plates adjacent thereto and effect an overlap of all joints formed between the abutting side edges of the front sections of said plates in relation to the joints formed between the abutting side edges of the rear sections of said plates.
 2. A microwave absorbing wall structure as defined in claim 1 wherein said plates have a regular hexagonal configuration and three adjacent sides of the front section thereof are offset from three adjacent sides of the rear section thereof.
 3. A microwave absorbing wall structure as defined in claim 1 wherein said plates have a square configuration and two aDjacent sides of the front section thereof are offset from two adjacent sides of the rear section thereof. 